1. Field of the Invention
The present invention applies to the field of performing spatial processing of received signals in radio communications systems and, in particular, to combining spatial processing, timing estimates and frequency offsets, to resolve a signal using a training sequence.
2. Description of the Prior Art
Mobile radio communications systems such as cellular voice and data radio systems typically have several base stations in different locations available for use by mobile or fixed user terminals, such as cellular telephones or wireless web devices. Each base station typically is assigned a set of frequencies or channels to use for communications with the user terminals. The channels are different from those of neighboring base stations in order to avoid interference between neighboring base stations. As a result, the user terminals can easily distinguish the transmissions received from one base station from the signals received from another. In addition, each base station can act independently in allocating and using the channel resources assigned to it.
Such radio communications systems typically include a broadcast channel (BCH). The BCH is broadcast to all user terminals whether they are registered on the network or not and informs the user terminals about the network. In order to access the network, a user terminal normally tunes to and listens to the BCH before accessing the network. It will then use the information in the BCH to request access to the network. Such a request typically results in an exchange of information about the network using separate control and access channels and ends in the user terminal receiving an assignment to a traffic channel TCH of a particular base station.
Frequency and timing offset or delay can sometimes be determined by a user terminal based on the BCH, and it will be more precisely defined during the registration and assignment process discussed above. In a spatial diversity multiple access system, the base station can enhance the capacity of the system by determining the position and range to the user terminal as well as any other spatial parameters. These spatial parameters are also better developed as TCH is assigned. However, all of the timing, frequency and spatial parameters are subject to drift and change over time and for optimum performance must constantly be updated.
In order to accurately resolve the TCH messages and determine spatial parameters for transmitting return traffic, the timing or delay, frequency offset and spatial signature of the user terminal""s message are all desired. Typically, it is preferred to accurately determine the timing or delay and frequency offset based on analyzing a long training sequence across a potentially long delay spread. This can create great demands on the processing resources of a base station and increase the amount of time required to generate a reply to the access request.
A method and apparatus are provided that performs spatial processing, timing estimation and frequency offset using a training sequence of a received burst. According to one aspect of the present invention, the invention includes receiving a burst having a known training sequence at a set of diversity antennas, sampling the received burst at each antenna, determining a coarse timing estimate for samples from at least one antenna, and determining a spatial weighting vector using the coarse timing estimate. The embodiment further includes applying the spatial weighting vector to the received burst samples for each antenna to form a single channel signal, determining a fine timing estimate for the single channel signal, determining a second spatial weighting vector using the fine timing estimate, applying the second spatial weighting vector to the received burst samples for each antenna to form a second single channel signal, and demodulating the second single channel signal.